n=3
def numTrees(n):
    dp=[0 for _ in range(n+1)]
    dp[0]=1
    dp[1]=1
    for i in range(2,n+1):
        for j in range(1,i+1):
            dp[i]+=dp[j-1]*dp[i-j]
    return dp[n]

print(numTrees(n))

